1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using a light source having a plurality of light emitting points. The present invention can be applied to an image forming apparatus such as a printer, a copying machine, a facsimile machine or a multifunction product thereof.
2. Description of the Related Art
In recent years, demand for accelerating an image forming rate and improving a resolution of an image forming apparatus has been escalating, and an increased scan rate is desired. To meet this demand, an image forming apparatus using a multibeam write optical scanning apparatus that scans one scan target surface relative to a plurality of beams is put to practical use. The optical scanning apparatus or particularly a laser scanning apparatus using a semiconductor laser is simple in structure, and high in scan rate and resolution. Due to this, the apparatus has been widely used in an image forming apparatus such as an image display apparatus or an image recording apparatus. The laser scanning apparatus is optimum as an exposure apparatus of an electrophotographic printer, and many products are circulated in the market, as laser printers.
The demand for accelerating the image forming rate and improving the resolution of the image forming apparatus has been further escalating and the increased scan rate is further desired. To realize a high optical scan rate, it is necessary to provide a high speed deflector. However, if the deflector is, for example, a rotating polygon mirror, there is an upper limit to the rotation speed of the mirror because of problems of noise, heat and the like. To solve these problems, there is provided an optical scanning apparatus using a vertical cavity surface emitting laser (hereinafter, “VCSEL”) including a plurality of light emitting regions (light emitting points or light emitting elements) that can be modulated independently of one another. This apparatus is applied to an image forming apparatus using a so-called multibeam scanning apparatus that emits a plurality of light fluxes from the VCSEL and that simultaneously scans a plurality of scanning lines by one scanning (Japanese Patent No. 3227226).
FIG. 1 is a perspective view of an optical scanning apparatus according to a first embodiment of the present invention. The outline of the optical scanning apparatus using the VCSEL that includes a plurality of light emitting regions will be explained with reference to FIG. 1. In FIG. 1, a light source 101 includes a plurality of light emitting regions 101a, and a light emitting control unit that controls operations of these light emitting regions. A coupling optical system 110 that includes a coupling lens 102, a cylindrical lens 103, and an aperture 104 is arranged on an optical path of a plurality of laser beams emitted from the light source 101. The laser beams emitted from the light source 101 are coupled to a subsequent optical system (an imaging optical system or a scanning optical system) by the coupling lens optical system 110 while cross-sectionally shaping the coupled laser beams by the aperture 104. The coupled laser beams are deflected and reflected by a deflector 105 constituted by a rotating polygon mirror. The coupled laser beams thus deflected and reflected are imaged on a scan target surface 108, which is an image surface, by a first scanning lens 106a and a second scanning lens 106b that constitute the scanning optical system. The scan target surface 108 is a surface of a cylindrical image carrier that supports a photoconductive medium. While the leaser beams are imaged on the scan target surface 108, a scanning line 109 is drawn by the deflection and reflection of the laser beams. A direction of the scanning line 109 corresponds to a main scanning direction. The image carrier is driven to rotate about its central axis, thereby moving the image surface in a direction at a right angle with respect to the main scanning direction. This moving direction corresponds to a sub-scanning direction. By thus performing optical scanning using the laser beams, an image is formed. In the example shown in FIG. 1, a first, a second, and a third mirrors 107a, 107b, and 107c that guide the deflected light fluxes to the scan target surface 108 are arranged.
If the VCSEL is used as the light source of a plurality of laser beams, an irregularity occurs among divergence angles of the beams. If the irregularity occurs, then sizes of optical spots formed on the scan target surface 108 by the respective beams differ, and dots that form the image have irregular sizes, resulting in a deterioration in graininess. Furthermore, for a color image forming apparatus that forms a color image by registering a plurality of toner images different in color, a misregistration occurs among dots in respective colors, thereby causing image quality degradations including a deterioration in color reproducibility. Japanese Patent No. 3227226 discloses a positional relationship between a scanning line and an image spot in the VCSEL, and the invention disclosed therein enables high speed scanning.
Furthermore, there is proposed a technique for setting a distance between the adjacent light emitting regions (light emitting points) of the VCSEL, reducing a thermal crosstalk, and realizing high density recording (see, for example, Japanese Patent Application Laid-open (JP-A) No. H10-301044). However, the conventional techniques including those disclosed in Japanese Patent No. 3227226 and JP-A No. H10-301044 do not solve the image quality degradations resulting from the characteristic irregularities among the respective light emitting regions of the VCSEL.
The VCSEL has the light emitting elements (light emitting regions) monolithically formed on the same substrate. Due to this, the irregularity occurs among distances between the adjacent light emitting regions, resulting from a technical problem in manufacturing the VCSEL. However, there is no avoiding allowing the irregularity within a certain range so as to improve manufacturing yield. If such an irregularity occurs, the VCSEL is unable to select and combine the laser beams having a coincident divergence angle, differently from the conventional semiconductor laser (LD). As a result, it is necessary to reduce the irregularity in beam spot diameter resulting from the irregularity among the divergence angles of the beams emitted from the light emitting elements (light emitting regions) included in one light source. It is also necessary to reduce an irregularity in light utilization efficiency.